class ArrayList implements List
{
 private Object[] myArray;

 ArrayList()
 {
  myArray = new Object[0];
 }

 private ArrayList(Object[] array)
 {
  myArray=array;
 }

 public boolean isempty()
 {
  return myArray.length==0;
 }

 public Object head()
 {
  return myArray[0];
 }

 public List tail()
 {
  Object[] array = new Object[myArray.length-1];
  for(int i=1;i<myArray.length;i++)
     array[i-1]=myArray[i];
  return new ArrayList(array);
 }

 public List cons(Object obj)
 {
  Object[] array = new Object[myArray.length+1];
  array[0]=obj;
  for(int i=0;i<myArray.length;i++)
     array[i+1]=myArray[i];
  return new ArrayList(array);
 }

 public static List empty()
 {
  return new ArrayList();
 }

 public String toString()
 {
  if(isempty())
     return "[]";
  else
     return "["+head()+restToString(tail());
 }

 private static String restToString(List L)
 {
  if(L.isempty())
     return "]";
  else
     return ","+L.head()+restToString(L.tail());
 }

}